Just as a follow-up to this for anyone who's interested:

With the graphics settings turned down the (single-core) box described above runs 4 instances of WoW without complaint. Laptop makes five

I'm currently levelling a Pala (bluekiera), Mage (bluekieran), Shammie (blukran), Priest (blukira) and Hunter (blukirran) on the Bloodfeather(EU)/Horde PvP server. They're at 11 so far & it's proving good fun! Incidentally, "make some trial accounts and give it a go" was an awesome piece of advice